1. Field of the Invention
This invention relates to an optical information processing apparatus such as an optical disk apparatus for causing the light beam from a light source to be condensed as a minute spot on a predetermined track on a recording medium and effecting recording or reproduction of information, and in particular to an apparatus provided with tracking means for correcting the positional deviation between the spot and the predetermined track.
2. Related Background Art
Generally in an optical disk apparatus of the DRAW (direct read after write) type or of the rewritable type, spiral guide grooves for tracking are preformed on the disk. The pitch of the guide grooves is as small as 1 .mu.m and therefore, when a light spot is applied to the guide grooves, a diffraction occurs and the diffracted light is scattered in a direction perpendicular to the tracks. In the tracking method called the push-pull method, the variation in the light and shade of the pattern of the O-order and .+-.1st-order diffracted lights on a detector for tracking is taken out as a tracking error signal, whereby tracking servo is effected.
In an optical disk apparatus wherein an objective for condensing a minute spot on a predetermined track is moved relative to a detector for tracking to thereby effect tracking servo if there is great eccentricity of the disk, the center of the spot on the detector is displaced to cause offset of the tracking error signal and thus, it has been difficult to accomplish accurate tracking.
This will be described in detail with reference to FIGS. 1A to 6 of the accompanying drawings.
FIGS. 1A,1B,2A and 2B schematically show the construction of an optical information processing apparatus according to the prior art. In these figures, reference numeral 1 designates a semiconductor laser, reference numeral 2 denotes a collimator lens, reference numeral 3 designates a half-mirror, reference numeral 7 denotes a disk, reference numeral 8 designates an actuator, reference numeral 9 denotes a condensing lens, and reference characters 10a and 10b designate the light-receiving surfaces of two-division detectors for tracking. In these figures, only optical systems necessary to explain the tracking method are shown and it is to be understood that detection of information carried on the disk and focus detection are accomplished in conventional manner.
In FIG. 1A, a light beam emitted from the semiconductor laser 1 is collimated by the collimator lens 2 and condensed on a predetermined track of the disk 7 having tracking guide grooves by the objective 6. Reference numeral 8 designates an actuator for moving the objective 6 in a direction perpendicular to the track as indicated by arrow T and crossing the optic axis of the incident light and correcting the positional deviation between the spot imaged by the objective and the track. When the eccentricity of the disk 7 is very small, the optic axis of the objective and the center of the light beam from the laser are substantially coincident with each other, and a diffracted light beam including the asymmetry of the .+-.1st-order diffracted lights corresponding to the positional deviation between the track and the spot again enters the objective 6. The light beam again collimated by the objective 6 is reflected by the half-mirror 3 and condensed on the light-receiving surfaces 10a and 10b of the detector by the condensing lens 9. The light receiving surfaces 10a and 10b are usually provided at a distance D from the Gaussian image surface of the condensing lens 9. The spot on the detector lies on the axis X--X' as shown in FIG. 1B, and a tracking error signal is produced by the asymmetry (not shown) of the .+-.1st-order diffracted lights on the light-receiving surfaces 10a and 10b.
A case where the eccentricity of the disk 7 is relatively great will now be described with reference to FIG. 2A. Assuming that the eccentricity is as great as .delta., the actuator 8 causes the objective 6 to follow it. Therefore, the light beam from the semiconductor laser 1 enters the objective with the center thereof deviating by .delta. from the center of the objective, so that if the focal length of the condensing lens is f.sub.2, the deviation .DELTA. of the center of the spot on the detector shown in FIG. 2B is given by the following equation: ##EQU1## For example, if .delta.=100 .mu.m and D=2 mm and f2=40 mm, .DELTA.=10 .mu.m, and this is not a small amount as compared with the diameter 250 .mu.m of the spot on the detector (when the focal length f.sub.1 of the objective is f.sub.1 =5 mm and the N.A. of the objective is N.A.=0.5)
In such as apparatus, a case where a tracking error signal 19 is obtained by the use of a circuit as shown in FIG. 3 of the accompanying drawings will now be considered. In FIG. 3, reference numerals 15 and 16 designate amplifiers for amplifying the outputs from the light-receiving surfaces 10a and 10b , respectively, and reference numeral 17 denotes a differential amplifier. When the eccentricity of the disk 7 is very small, the center of the objective and the center of the light beam from the laser are substantially coincident with each other as previously described and therefore, when servo has been effected so as to render the tracking error signal 19 zero, .+-.1st-order diffracted lights 20 and 21 are diffracted symmetrically with respect to the track as shown in FIG. 4A of the accompanying drawings, and the spot accurately follows a predetermined track. The light amount distribution on the detector is such as shown in FIG. 4B of the accompanying drawings. Reference numerals 22 and 24 designate areas in which .+-.1st-order diffracted lights overlap O-order diffracted light 23.
When the eccentricity of the disk 7 is relatively great, the center of the spot deviates by .DELTA. from the optic axis on the detector as shown in FIG. 5B of the accompanying drawings and therefore, when servo has been effected so as to render the tracking error signal 19 into, .+-.1st-order diffracted lights 25 and 26 are diffracted asymmetrically with respect to the track as shown in FIG. 5A of the accompanying drawings, and the spot causes track deviation of d from the center of a predetermined track. In such a case, crosstalk with the adjacent tracks increases to aggravate the error rate.
To overcome such a disadvantage, an apparatus of the construction as shown, for example, in FIG. 6 of the accompanying drawings is proposed. That is, a beam splitter 30 is inserted between a half-mirror 3 for separating the optical paths of the light beam from a light source and a light beam travelling toward reproduction signal detecting means and an objective 6 to direct part of the reflected light beam from a disk to the light-receiving surfaces 10a and 10b of a detector for tracking. All these are carried by the same housing 31 and therefore, the spot on the detector does not move even if tracking is effected. Such an apparatus, however, suffers from the following disadvantages:
(1) The construction is complicated and this is disadvantageous with respect to cost.
(2) The weight of the movable portion is increased and an actuator 8 is bulky.
(3) The presence of the beam splitter 30 reduces the utilization efficiency of the light amount.
On the other hand, in order to correct the offset of the tracking error signal as previously described, U.S. Pat. No. 4,302,830 proposes an optical information processing apparatus provided with means for detecting the position of the objective. This apparatus is designed such that a piezo-electric element is attached to a resilient member for movably supporting the objective and with movement of the objective, the piezo-electric element is deformed with the resilient member to thereby produce an electrical signal, from which the position of the objective is detected.
However, in the apparatus described in the aforementioned U.S. Pat. No. 4,302,830 the piezo-electric element must be deformed simultaneously with the movement of the objective and therefore, the force of restitution of this piezo-electric element provides a load and thus, it has been impossible to move the objective by a small drive force. Also, it has been conceived that if the apparatus is used for a long period of time, repeated deformation of the piezo-electric element fatigues and destroys it.